The Radio Frequency (RF) spectrum extends from low frequency radio, through radio, microwave, terahertz, infra-red, visible, ultra-violet, and finally x-rays, and while the fundamental form of all the waves are the same, the mechanism for formation, and absorption of each varies. Radio and microwaves are created by macroscopic currents flowing or oscillating through a bulk material—as in a semiconductor, or and antenna. Much of the infra-red, Visible and Ultra-violet spectrum corresponds to energies attained through quantum electron shifts, and vibrational resonances of molecules. Very high energy and frequency radiation, such as X-rays, are produced by high energy particle interactions, such as bremstrahlung, or ionization of inner electrons. However there exists a gap between the maximum capabilities of current electronics, and the low end of the IR spectrum—this is the so-called terahertz gap, as it lies between ˜300 GHz and 3 THz.
Terahertz (THz) radiation, also referred to as T-rays, Far-Infrared (FIR), and sub-millimeter radiation, is both interesting, and potentially very useful, as it has certain properties normally associated with both visible light and microwaves. For example, THz has the ability to pass through most materials with little attenuation. It has high attenuation in water, and is unable to penetrate metals. Yet, because of its lower wave length, it behaves more as a ray, and less as a wave, at small scales. The short wavelength 1 mm (300 Ghz) to 0.1 mm (3 THz) also allows for higher resolution than is attainable with microwave radiation. (This can be derived directly from the Rayleigh criterion.)
Groups have shown that THz can be used for tooth/bone density investigations, to detect cavities before they form [2]. The same technique, called Terahertz Time Domain Spectroscopy, has been used by Mittelman [3] to measure the position of raisins within a box of cereal, and density variation in chocolate bars. Various space agencies are also keenly interested in THz radiation, as much of the inter-stellar dust in the universe radiates in the THz region. The same technology is useful for study of the upper atmosphere. Many chemicals have unique absorption and transmission characteristics in the THz band—so a THz source would be immensely useful in chemical analysis and spectroscopy. A space mounted version of this would allow researchers to track pollutants in the upper atmosphere.
There is also interest in T-rays in the homeland security and intelligence community. T-rays can penetrate most materials easily, such as fabrics, plastics and cardboard. But unlike X-rays, they are non-ionizing radiation, so there are no obvious health concerns. This enables terahertz radiation to be used on biological specimens, and even people. Additionally, the spectroscopic data mentioned above could be use to chemically fingerprint explosives and other malicious agents. The resolution is so good that it is possible to read un-opened letters, based on the chemical signature of the ink used.
Despite the obvious potential of THz radiation, sources of T-rays remain either very expensive, very low power, or both. There have been several general approaches. The first involves non-linear reactive coupling of lower frequency signals. This takes advantage of available components operating in the 100 GHz range (which are scarce), and various materials with non-linear optical properties. High order multipliers are inefficient, so many THz sources use stacked low order frequency multipliers. This results in a signal with power of a few hundred μW.
High power THz radiation has also been produced through the use of a free electron laser (FEL), which operates by sending a relativistic electron through a sinusoidal magnetic field. From the electron's perspective, the magnetic field appears as a virtual photon, so can undergo Compton scattering. By controlling the frequency of the magnetic field (wavelength, and electron velocity) the frequency of the resulting radiation can be controlled, and tuned to THz frequencies. While this yields high power radiation, it requires millions of dollars of investment, and a large facility, of which only a handful exist in the world.
A similar technology is under research by Walsh at Dartmouth [6], and with Vermont Photonics. Walsh and his colleagues created a repetitive structure, capable of supporting a standing electromagnetic wave (often called a slow wave structure). By passing relativistic electrons across the surface of the slow wave structure, the electrons are effectively traveling faster than the speed of light. Superluminal electrons slow their velocity through an electromagnetic equivalent of a shockwave, called Cerenkov radiation. This is the same radiation that accounts for the blue glow associated with pool-type nuclear reactors. By controlling the electron velocity, and the slow wave structure, it is possible to tune the frequency of the Cerenkov radiation to the THz region. Vermont Photonics has achieved powers of up to 450 μW, using this technology [8].
Another technique that has been commercially successful because of its affiliation with THz Time Domain Spectroscopy involves down-conversion of optical wavelengths, through a photo-electric crystal. A dipole antenna is charged, and coupled by a photo-active element. A femto-second pulsed laser is then shone on the photoconductor, producing a THz pulse. Opto-electronic terahertz sources require a very well controlled laser source (Colliding pulse mode-locked, or CO2). Efficiency continues to be very low, as a 120 W laser system can produce only a few mW of THz energy [2] [4]. Both these methods involve altering higher, or lower frequency radiation to produce THz frequencies. There exists another class of devices which operate by amplifying a signal. A band pass filter, and a feedback loop can allow these devices to operate as oscillators. Solid state signal sources are limited by material properties at high frequencies, and therefore have difficulties exceeding 100 GHz. However, the precursor to solid state electronics—vacuum tubes—can often offer higher performance than solid state devices, allowing them to operate at higher powers or higher frequencies.
Vacuum electronic technology originated with the first computers, radar, and generally the first RF systems. In the most basic form, a vacuum tube uses the coupling between electric forces, and kinematics of electrons to introduce gain to an RF signal.
While generally more expensive than solid state electronics, there are several applications in which vacuum electronics are regularly found. High power applications, such as TV or radio transmission towers and even satellite transmission often use Klystrons, Twystrons, Twystrodes, or other power vacuum tubes. Most particle accelerators use vacuum tubes to generate the accelerating fields. Many military applications also use vacuum electronics because they can operate in a wider temperature range, and are less sensitive to electronic noise (Hardened electronics).
The operating frequency of a vacuum device is dependent on the size and geometry of the resonating elements. In the case of a klystron, the resonators are simple cavity resonators. A traveling wave tube (TWT) employs a helical coil or a serpentine wave guide to support a wave (another slow wave structure). In each case, the resonant frequency is governed by the geometry, and is of a scale on the order of the wavelength. This makes conventional power vacuum electronics effective for frequencies ranging from about 5 MHz up to 25 GHz.
Several attempts are currently under way to produce high frequency vacuum tubes by using a variety of microfabrication techniques. Much of the focus is on providing power for a next generation linear accelerator—which necessitates a power source of several mega-watts operating in the W-band (about 95 GHz).
The Stanford Linear Accelerator Center (SLAC) developed a 94 GHz device, called a Klystrino, using a microfabrication technique known as LIGA (from the German acronym for lithography, electroplating and micro-molding) [7]. Because of its application, it was designed to produce 400 kW peak power, and 4 kW average power, through and assembly of 4 individual beam tunnels [9]. The device was successfully fabricated, and assembled, and the cavity properties were measured while beam loaded. It required an overall precision of ˜5 μm, which was achieved with LIGA for the cavities, and wire EDM for the drift tube. It ultimately failed under test due to a magnetic misalignment. [10] LIGA has also been used for fabrication of resonant cavities for accelerator purposes. J. J. Song et al. [11] made a sample accelerator cavity, 7 cm long with an operating frequency of 94 GHz, as well as designs for a 1-m long structure with an operating frequency of 108 GHz. The individual cavities are on the order of 1 mm, but the tolerance is <0.2%, to attain the required Q. LIGA seems to have a lot of potential; however it is limited in its use, because it requires a hard x-ray source for exposure, such as a synchrotron, and long exposure times (4-8 hours). As such, waiting lists for LIGA exposure are months or even years long.
The Jet Propulsion Lab has also fielded a project, led by Manohara [12] to produce a reflex klystron that operates at 1.2 THz, and produces 3 mW of power with an efficiency of 0.2%. Manohora and his colleagues propose etching a gridded cavity using deep reactive ion etching (DRIE), and silicon bonding. Several proposals have been produced, and various components have been fabricated, however no functional device has been produced. The THz reflex klystron is designed to operate with a large current (3 mA), which requires a cathode current density of 100 to 1000 A/cm2. While Manohora and his colleagues at the Jet Propulsion Lab propose using a carbon nanotube field emission array (FEA), they have not succeeded in creating such a device.
A private research company, Calabazas Creek Research Inc. has designed a backwards wave oscillator (BWO) using a variety of microfabrication techniques, including electric discharge machining (EDM) and LIGA. The device operates from 500-600 GHz, and is expected to produce 6-8 mW of power. Backward wave oscillators offer more tunability than klystrons, but give up efficiency. The design includes a reservoir type thermionic cathode, and a depressed collector, both of which act to increase the efficiency of the device. The structure has been fabricated using LIGA, however only 8% of the fabricated circuits were in usable condition. No results from tests were found. [13]
Similar projects are in progress at Northrop Grumman, and Genvac. The Northrop Grumman project [14] uses a serpentine waveguide to act as a slow-wave structure, like a TWT. It has been successfully fabricated, and was able to produce a few milliwatts of power with an efficiency of 0.6% at 650 GHz. The Genvac approach [15] also uses slow wave structures, however it employs a fabrication process using chemical vapor deposition (CVD) diamond for structural elements. The resonant structure has been fabricated, but successful operation has not yet been demonstrated.
As such, there still exists a need in the art for improved vacuum compatible THz frequency electromagnetic wave sources, methods of micro-fabricating such devices and resonant cavities suitable therefore.